Orthopedic Basic Science 2026 MCQs: Board Review Part 4

Question 1

Osteopenia is defined by the World Health Organization (WHO) as a bone mineral density (BMD) that is

Explanation

Osteopenia, decreased bone mass without fracture risk as defined by the WHO criteria for diagnosis of osteoporosis, is when a woman's T-score is within -1 to -2.5 SD. The T-score represents a comparison to young normals or optimum peak density. The Z-score represents a comparison of BMD to age-matched normals. Measurements of bone mineral density (BMD) at various skeletal sites help in predicting fracture risk. Hip BMD best predicts fracture of the hip, as well as fractures at other sites.

Question 2

Which of the following best describes the mechanism of action of gentamycin?

Explanation

Gentamycin and the aminoglycosides (ie, streptomycin, tobramycin, amikacin, and neomycin) work by binding to the 30s ribosome subunit, leading to the misreading of mRNA. This misreading results in the synthesis of abnormal peptides that accumulate intracellularly and eventually lead to cell death. These antibiotics are bactericidal. Cephalosporins, vancomycin, and penicillins interfere with cell wall synthesis by inhibiting the transpeptidase enzyme. Polymyxin, nystatin, and amphotericin increase cell membrane permeability by disrupting the functional integrity of the cell membrane. The quinolones inhibit the enzyme, DNA gyrase. Lastly, metronidazole forms oxygen radicals that are toxic to anaerobic organisms because they lack the protective enzymes, superoxide dismutase and catalase.

Question 3

What type of muscle contraction occurs while the muscle is lengthening?

Explanation

A muscle that lengthens as it is activated is an eccentric contraction. Isometric contraction involves no change in length. Concentric contraction occurs while the muscle is shortening. In isotonic contraction, the force remains constant through the contraction range. Isokinetic muscle contraction occurs at a constant rate of angular change of the involved joint. Garrett WE, Speer KP, Kirkendall DT (eds): Principles & Practice of Orthopaedic Sports Medicine. Philadelphia, PA, Lippincott Williams & Wilkins, 2000, pp 12-13.

Question 4

Osteoclasts originate from which of the following cell types?

Explanation

Osteoclasts originate from the monocyte/macrophage lineage. Fibroblasts and osteoprogenitor cells originate from mesenchymal stem cells and do not form osteoclasts. Plasma cells reside in the bone marrow and are derivatives of the hematopoietic system. Megakaryocytes are also in the bone marrow and synthesize platelets. Zaidi M, Blair HC, Moonga BS, et al: Osteoclastogenesis, bone resorption, and osteoclast-based therapeutics. J Bone Miner Res 2003;18:599-609. Brinker MR: Bone (Section 1), in Miller M (ed): Review of Orthopaedics, ed 2. Philadelphia, PA, WB Saunders, 1996, pp 1-35.

Question 5

A study is being designed to compare the effectiveness of an antibiotic. The choice of the number of patients (ie, the sample size) depends on several factors. What type of calculation assesses the potential of the study to successfully address the effectiveness of the antibiotic?

Explanation

Power analysis is used to determine the minimum number of specimens (sample size) such that, if a difference is found that is large enough to be clinically important, the associated level of statistical reliability will be high enough (ie, the P-value will be small enough) for the investigators to conclude that the difference observed in the study also holds in general. For the statistician to do a power analysis, the investigators must first decide on the minimum difference that they consider to be clinically important, for example, a reduction of 3% in the rate of infection. It is important to recognize that the choice of what constitutes the minimum difference in the rate of infection that is clinically (ie, medically) important cannot and should not be done by the statistician. Rather, this is a clinical-medical issue and must be done by the physician researcher based on a comprehensive assessment of the medical risks and benefits. The power analysis also requires an estimate of the variance in the data, which may be based on previous similar studies, if available. A statistician can then calculate the minimum sample size (number of patients) required such that, if a clinically important difference does, in fact, exist between the full populations, there is a reasonable probability or power (typically 80% to 90%) that a difference this large also will occur between the sample populations at the desired level of statistical significance (usually, but not necessarily, P < 0.05). The other answers refer to types of analyses that are usually conducted after the data are collected.

Question 6

What is the most common cause of mechanical failure of an orthopaedic biomaterial during clinical use?

Explanation

In most orthopaedic applications, the materials are strong enough to withstand a single cycle of loading in vivo. However, these loads may be large enough to initiate a small crack in the implant that can grow slowly over thousands or millions of cycles, eventually leading to gross failure. Such fatigue failure has occurred with virtually every type of implant, including stainless steel fracture plates and screws, bone cement in joint arthroplasty, and polyethylene inserts in total knee arthroplasty. Lewis G: Fatigue testing and performance of acrylic bone-cement materials: State-of-the-art review. J Biomed Mater Res Br 2003;66:457-486. Stolk J, Verdonschot N, Huiskes R: Stair climbing is more detrimental to the cement in hip replacement than walking. Clin Orthop 2002;405:294-305.

Question 7

Which of the following body positions is associated with the highest intradiskal pressure?

Explanation

Intradiskal pressure is lowest when the patient is in the supine position. Sitting is associated with higher intradiskal pressures than standing. Flexion also increases intradiskal pressure. The combination of flexion and sitting produces the highest intradiskal pressure. Nachemson and Morris found that intradiskal pressure increases as position changes from lying supine, lying prone, standing, leaning forward, sitting, and sitting leaning forward. Twisting or straining in positions of relatively high intradiskal pressure may predispose patients to herniation of the intervertebral disk. Patients with a herniated disk may also notice their pain worsens with activities that increase the disk pressure, including the positions mentioned, or activities that increase intra-abdominal pressure (coughing, sneezing, straining). Nachemson A, Morris JM: In vivo measurements of intradiscal pressure. J Bone Joint Surg Am 1964;46:1077-1092.

Question 8

Figure 6 shows an object being held in an outstretched hand. To offset the moment created by the object (ignoring the weight of the forearm), the biceps must generate a force of

Explanation

Answering this question requires understanding of two important biomechanics concepts. First, because neither the object being held in the hand nor the body is moving and, hence, their accelerations are zero, the problem is one of static equilibrium in which the sum of the moments acting on the body is zero. Second, a moment is the action of a force that causes an object to rotate about any point away from its line of action. The magnitude of the moment is the magnitude of the force multiplied by the perpendicular distance between the line of action and the point (often called the moment arm or lever arm). In this problem, two forces are causing moments about the elbow. The magnitude of the moment caused by the object in the hand is 5 N times 30 cm or 150 N-cm. To maintain equilibrium, the moment caused by the biceps force must also have a magnitude of 150 N-cm. Its moment arm is 2 cm, so the magnitude of the biceps force is 150 N-cm divided by 2 cm, which equals 75 N. In general, functional loads such as the object are always at a mechanical advantage (ie, have a longer moment arm) over the muscle. Therefore, muscles must generate large forces to overcome the moments caused by even small functional loads. An KN, Chao ES, Kaufman KR: Analysis of muscle and joint loads, in Mow VC, Hayes WC (eds): Basic Orthopaedic Biomechanics, ed 2. New York, NY, Lippincott-Raven, 1997, pp 1-14.

Question 9

Which of the following best describes the function of the notochord?

Explanation

The notochord is the anatomic structure that defines the phylum Chordata. The notochord plays a fundamental role in the development of the skeleton, and it exists only for a short period of time. During its temporary existence, the notochord serves as a transient axis of support, provides for the initial axis of orientation of the developing embryo, and most importantly, plays a vital role in the induction of the tissues that eventually form the vertebral column.

Question 10

A patient undergoes a proximal tibial resection that is reconstructed with a fresh frozen osteoarticular allograft. Eleven months later, the graft is retrieved. Histologically, the articular cartilage and subchondral bone retrieved would be expected to show evidence of

Explanation

Osteoarticular allografts are devoid of host chondrocytes but do contain "mummified" cellular debris left over from donor processing. The cartilage architecture is preserved in the first 2 to 3 years after transplantation. The articular surface is covered with a pannus of fibrocartilage maintaining the joint space radiographically; this pannus later contains islands of fibrocartilage containing host mesenchymal stem cells. Degenerative changes to the joint surface occur earlier and are more severe in joints that are unstable. Only with degenerative changes at the surface is there histologic evidence of subchondral revascularization. Often degenerative changes involving the articular cartilage reach the tidemark, but the tidemark itself remains structurally intact. Enneking WF, Campanacci DA: Retrieved human allografts: A clinicopathological study. J Bone Joint Surg Am 2001;83:971-986.

Question 11

Stiffness relates the amount of load applied to a structure like a long bone or an intramedullary nail to the amount of resulting deformation that occurs in the structure. What is the most important material property affecting the axial and bending stiffness of a structure?

Explanation

The amount of deformation resulting in response to an applied load depends on the stress distribution that the load creates in the structure and the stress versus strain behavior of the material that makes up the structure. Axial and bending loads create stress distributions that involve normal stresses and normal strains. Although all five responses are indeed material properties, only one, elastic modulus, relates normal stresses to normal strains. In fact, axial and bending stiffness are directly proportional to modulus, so that a nail made from stainless steel will have nearly twice the stiffness of a nail made from titanium alloy (because their respective elastic moduli differ by about a factor of two). Hayes WC, Bouxsein ML: Analysis of muscle and joint loads, in Mow VC, Hayes WC (eds): Basic Orthopaedic Biomechanics, ed 2. New York, NY, Lippincott-Raven, 1997, pp 74-82. Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 159-165.

Question 12

Which of the following accurately defines changes in Vitamin D requirements as the result of aging?

Explanation

Older individuals ingest less Vitamin D and are unable to generate as much as younger people via the skin in response to ultraviolet exposure; thus, there is a decrease in the levels of serum 25(OH) D. This reduction in 25(OH)D leads to a reduction in calcium absorption. There is also decreased conversion in the kidney of 25(OH)D to 1,25(OH)D. This all leads to an increase in the daily requirements of both calcium and Vitamin D. It also results in a responsive increase in PTH secretion in the elderly, as well as renal function impairment and possible renal resistance to PTH. Dawson-Hughes B, Harris SS, Krall EA, et al: Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age and older. N Engl J Med 1997;337:670-676. Recker RR, Hinders S, Davies M, et al: Correcting calcium nutritional deficiency prevents spine fractures in elderly women. J Bone Miner Res 1996;11:1961-1966.

Question 13

Osteoclasts are primarily responsible for bone resorption of malignancy. Which of the following stimulates osteoclast formation?

Explanation

Bone destruction is primarily mediated by osteoclastic bone resorption, and cancer cells stimulate the formation and activation of osteoclasts next to metastatic foci. Increasing evidence suggests that receptor activator of NF-kB ligand (RANKL) is the ultimate extracellular mediator that stimulates osteoclast differentiation into mature osteoclasts. In contrast, OPG inhibits osteoclast development. IL-8 but not IL-5 is known to play a role in osteoclastogenesis. MMP-2 and collagen type I do not have a direct role in osteoclastogenesis. Kitazawa S, Kitazawa R: RANK ligand is a prerequisite for cancer-associated osteolytic lesions. J Pathol 2002;198:228-236.

Question 14

Which of the following statements best describes synovial fluid?

Explanation

Synovial tissue is composed of vascularized connective tissue that lacks a basement membrane. Two cell types (type A and type B) are present: type B cells produce synovial fluid. Synovial fluid is made of hyaluronic acid and lubricin, proteinases,and collagenases. It is an ultrafiltrate of blood plasma added to fluid produced by the synovial membrane. It does not contain erythrocytes, clotting factors, or hemoglobin. It lubricates articular cartilage and provides nourishment via diffusion. Synovial fluid exhibits non-Newtonian flow characteristics. The viscosity coefficient is not a constant, the fluid is not linearly viscous, and its viscosity increases as the shear rate decreases.

Question 15

Collagen orientation is parallel to the joint surface in what articular cartilage zone?

Explanation

The collagen orientation changes from parallel in the superficial zone to a more random pattern in the middle zone and finally to perpendicular in the calcified zone. Koval KJ (ed): Orthopaedic Knowledge Update 7. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 498-499.

Question 16

Which of the following agents increases the risk for a nonunion following a posterior spinal fusion?

Explanation

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been shown to increase the risk of pseudarthrosis. In a controlled rabbit study, nonunions were reported with the use of toradol and indomethacin. NSAIDs are commonly used medications with the potential to diminish osteogenesis. Studies clearly have demonstrated inhibition of spinal fusion following the postoperative administration of several NSAIDs, including ibuprofen. Cigarette smoking is another potent inhibitor of spinal fusion. Glassman SD, Rose SM, Dimar JR, et al: The effect of postoperative nonsteroidal anti-inflammatory drug administration on spinal fusion. Spine 1998;23:834-838.

Question 17

What is the primary benefit of using rhBMP-2 instead of autogenous bone graft inside an anterior spinal fusion cage?

Explanation

Radiographic fusion success rates are best described as slightly rather than substantially improved. Hospital stay and time to fusion also are not substantially decreased. Use of BMP does eliminate the need for autograft harvest for anterior lumbar interbody fusion/cage. The need for postoperative bracing is not altered based on graft choice. Early studies of posterolateral fusion applications appear to show a greater potential benefit of rhBMP-2 over autograft. Not all BMP formulations perform the same with regard to safety and effectiveness. Other osteoinductive proteins in clinical trials (BMP-7, GDF-5) will require individual analysis of these properties.

Question 18

Cell signaling through the activation of a transmembrane receptor complex formed by serine/threonine kinase receptors occurs with which of the following growth factors?

Explanation

Cell activation and transcription varies with the target cell, the growth factor-receptor combination, and the biologic state of the cell. The growth factors in the transforming growth factor-beta (TGF-ß) superfamily signal through serine/threonine kinase receptors. Fibroblast growth factors, insulin-like growth factors, and platelet-derived growth factors signal through tyrosine kinase receptors. Growth hormone is released by the pituitary and circulates to the liver where target cells are stimulated to release insulin-like growth factor. Lieberman J, Daluiski A, Einhorn TA: The role of growth factors in the repair of bone: Biology and clinical applications. J Bone Joint Surg Am 2002;84:1032-1044.

Question 19

A 67-year-old woman has persistent anterior thigh and knee pain after undergoing total knee arthroplasty 1 year ago. Examination and radiographs reveal no problems in the knee, mild hip flexor weakness (grade 4+), and decreased sensation over the anterior thigh including and proximal to the incision. MRI of the lumbar spine will most likely reveal which of the following findings?

Explanation

Degenerative spondylolisthesis at L3-4 is the most likely diagnosis. This spondylolisthesis would result in foraminal stenosis affecting the third lumbar root and leading to anterior thigh and knee pain and hip flexor weakness. L4-5 spondylolisthesis would impinge on the L4 root in the foramen. Degenerative disk disease without hypertrophy is unlikely to have root impingement. Posterolateral herniations typically affect the inferior root and are less common in this age group. Hoppenfeld S: Physical Examination of the Spine and Extremities. Upper Saddle River, NJ, Prentice Hall, 1976, p 250.

Question 20

Decreased activity of which of the following proteins may be predictive of an aggressive soft-tissue sarcoma?

Explanation

It has been proposed that an imbalance in the proteolytic cascade involving matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) may play a role in the development or progression of malignancy. TIMP activity has been shown to be weak or nonexistent and MMP activity to be high in patients with soft-tissue sarcoma. The level of vascular endothelial growth factor is shown to be a negative prognostic indicator because the expression of this factor is associated with angiogenesis and aggressive growth of many tumors including Ewing' s sarcoma and chondrosarcoma. Stromelysin is a type of MMP. The biology of cytokines in malignancy is complex. A low level of interleukin-1 is not known to be associated with high-grade tumors.

Question 21

Passive glycation of articular cartilage results in

Explanation

Passive glycation of articular cartilage occurs over decades. One of the consequences of this glycation appears to be the stiffening of collagen. This phenomenon appears to be associated with an increased collagen degradation and development of osteoarthrosis. Passive glycation also results in a relatively yellow appearance. Passive glycation does not directly influence chondrocyte proliferation. DeGroot J, Verzijl N, Wenting-van Wijk MJ, et al: Accumulation of advanced glycation end products as a molecular mechanism for aging as a risk factor in osteoarthritis. Arthritis Rheum 2004;50:1207-1215.

Question 22

A study was conducted in 500 patients to measure the effectiveness of a new growth factor in reducing healing time of distal radial fractures. The authors reported that average healing time was reduced from 9.2 to 8.9 weeks (P < 0.0001). Because the difference was highly statistically significant, they recommended routine clinical use of this drug despite its high cost. A more appropriate interpretation of these results is that they are

Explanation

The results are statistically significant (at the arbitrary level of P < 0.05). That is, they indicate a probability of only 1/10,000 that the observation that the drug is effective in reducing healing time by 0.3 weeks occurred by chance selection of the study subjects. However, because the statistical power of a study increases with the number of subjects included (sample size), a difference that is trivial clinically can occur with a very high level of statistical significance (a very small P-value) if enough patients are included in the study. Because of this, the P-value alone, no matter how small, does not establish clinical significance or importance. Rather, the clinical significance of the observed difference must be assessed taking into consideration the medical importance of the difference if it is, in fact, true in the general population. In this example, the reduction in healing time of only a few days is probably clinically unimportant, particularly if the use of the new growth factor is expensive, complex, and/or has substantial side effects.

Question 23

In patients with suspected hepatitis C, which of the following tests is commonly used to confirm the diagnosis after a positive ELISA screening test?

Explanation

The basic diagnostic test for hepatitis C (HCV) is detection of an antibody to epitopes on an enzyme-linked immunosorbent anti-HCV assay (ELISA). The currently used ELISA has high sensitivity (92%) and specificity (95%). False positives, however, still occur. The currently used supplemental test for HCV is strip immunoblot assay, which is based on detection of several HCV epitopes on nitrocellulose paper by antibody-capture techniques. Molecular amplification by PCR technology is very sensitive, but difficult to standardize and susceptible to contamination. Microarray and proteomics are relatively recent molecular techniques used for analysis of genes or proteins, respectively. A Northern blot is used to detect mRNA levels of specific genes but is not used in this situation. de Medina M, Schiff ER: Hepatitis C: Diagnostic assays. Semin Liver Dis 1995;15:33-40.

Question 24

What type of multiple lesions is associated with Maffucci's syndrome?

Explanation

Maffucci's syndrome is a form of enchondromatosis associated with subcutaneous and deep hemangiomas. Similar to Ollier's disease, the risk of malignant transformation of the enchondromas is much higher than that of a solitary enchondroma. Multifocal nonossifying fibromas associated with other clinical findings such as mental retardation and café-au-lait spots is known as Jaffe-Campanacci syndrome. There are two types of multifocal forms of histiocytosis: Letterer-Siwe and Hand-Schüller-Christian disease. Schwartz HS, Zimmerman NB, Simon MA, et al: The malignant potential of enchondromatosis. J Bone Joint Surg Am 1987;69:269-274. Frassica F: Orthopaedic pathology, in Miller M (ed): Review of Orthopaedics, ed 2. Philadelphia, PA, WB Saunders, 1996, pp 292-335.

Question 25

Joint contact pressure in normal or artificial joints can best be minimized by what mechanism?

Explanation

Joint contact pressure is a stress and as such is defined as the load transferred across the joint divided by the contact area between the joint surfaces (the area over which the joint load is distributed). Therefore, any mechanism that decreases the load across the joint (eg, a walking aid) will decrease the stress. Similarly, any mechanism that increases the area over which the load is distributed (eg, using a more conforming set of articular surfaces in a knee joint arthroplasty) will also decrease the stress. Other mechanisms that influence joint contact pressure include the elastic modulus of the materials (cartilage in the case of natural joints and polyethylene in joint arthroplasty) and the thickness of the structures through which the joint loads pass. Bartel DL, Bicknell VL, Wright TM: The effect of conformity, thickness, and material on stresses in UHMWPE components for total joint replacement. J Bone Joint Surg Am 1986;68:1041-1051.

Question 26

Romosozumab is a monoclonal antibody utilized in the treatment of severe osteoporosis. What is the precise molecular target and subsequent downstream effect of this medication?

Explanation

Romosozumab is a humanized monoclonal antibody that targets and inhibits sclerostin. Sclerostin is naturally secreted by osteocytes and normally inhibits the Wnt/beta-catenin pathway by binding to LRP5/6 receptors on osteoblasts. By inhibiting sclerostin, Romosozumab disinhibits this pathway, leading to a dual effect: substantially increased osteoblast activity (anabolic) and decreased osteoclast activity (anti-resorptive), thereby increasing bone mineral density.

Question 27

A surgeon places a stainless steel screw through a titanium plate. Which of the following best describes the resulting electrochemical interaction between these two implants?

Explanation

When two dissimilar metals are placed in an electrolytic solution (such as body fluid) and are in electrical contact, galvanic corrosion occurs. The less noble metal (stainless steel) acts as the anode and preferentially corrodes, while the more noble metal (titanium) acts as the cathode and is protected. To prevent this, mixing dissimilar metals in direct contact should generally be avoided in orthopedic surgery.

Question 28

Examine the following illustration of articular cartilage.

Which of the following characteristics accurately describes the superficial (tangential) zone of articular cartilage compared to the deeper zones?

Explanation

The superficial (tangential) zone of articular cartilage makes up about 10-20% of the cartilage thickness. It has the highest concentration of water (approximately 80%), the highest concentration of collagen (primarily type II), and the lowest concentration of proteoglycans. The collagen fibers are oriented parallel to the joint surface to resist shear forces. Chondrocytes in this zone are flattened, rather than spherical or arranged in columns (which is characteristic of the deep zone).

Question 29

A 5-year-old boy presents with disproportionate short stature, a very short trunk, and a barrel chest. Radiographs reveal flattened vertebral bodies (platyspondyly) and delayed ossification of the epiphyses. His limbs are relatively proportional to his trunk, unlike classic dwarfism. A mutation in which of the following genes is most likely responsible for this condition?

Explanation

The patient's presentation of disproportionate short stature with a short trunk, barrel chest, and platyspondyly is classic for Spondyloepiphyseal Dysplasia Congenita (SEDC). SEDC is caused by an autosomal dominant mutation in the COL2A1 gene, which encodes type II collagen (crucial for normal cartilage formation). FGFR3 mutations cause achondroplasia (characteristically short limbs, relatively normal trunk). COMP mutations cause Multiple Epiphyseal Dysplasia or Pseudoachondroplasia. COL1A1 causes Osteogenesis Imperfecta. RUNX2 causes Cleidocranial Dysplasia.

Question 30

In the pathophysiology of aseptic loosening following total hip arthroplasty, the most robust inflammatory macrophage response and subsequent osteolysis is triggered by ultra-high molecular weight polyethylene (UHMWPE) wear particles of which specific size range?

Explanation

Macrophage activation and the subsequent inflammatory cascade (release of TNF-alpha, IL-1, IL-6) leading to osteolysis is most efficiently triggered by submicron polyethylene particles, specifically those in the 0.1 to 1.0 micrometer range. Particles in this size range are easily phagocytosed by macrophages, initiating an intracellular cascade that fails to digest the particle, keeping the macrophage continually activated. Particles larger than 10 micrometers are typically surrounded by foreign-body giant cells rather than individually phagocytosed.

Question 31

Review the provided histological image demonstrating a multi-nucleated cell actively resorbing bone.

Which enzyme is primarily responsible for generating the acidic environment within the ruffled border (Howship's lacuna) required to dissolve the inorganic mineral phase of the bone?

Explanation

The image depicts an osteoclast in a Howship's lacuna. Osteoclasts dissolve the inorganic hydroxyapatite component of bone by creating an acidic environment. This is achieved by the intracellular enzyme Carbonic anhydrase II, which converts CO2 and H2O into H+ and HCO3-. The H+ is then pumped into the resorption pit via a vacuolar H+-ATPase. After the mineral phase is dissolved, enzymes like Cathepsin K and MMPs are secreted to degrade the organic collagenous matrix.

Question 32

During the late swing phase of the normal gait cycle, the hamstrings undergo an eccentric contraction to decelerate the advancing tibia. Which of the following proprioceptive organs is primarily stimulated by this excessive lengthening, acting to reflexively induce muscle contraction and prevent overstretching?

Explanation

Muscle spindles are specialized stretch receptors located within the muscle belly that detect changes in muscle length and the rate of change in length. When a muscle is stretched (lengthened), the muscle spindle is activated and sends afferent signals (via Ia fibers) to the spinal cord, inducing a reflex contraction (myotatic reflex) to prevent overstretching. Golgi tendon organs, in contrast, sense muscle tension (via Ib afferents) and act to inhibit the muscle to prevent avulsion or tearing.

Question 33

A patient develops a surgical site infection caused by Methicillin-Resistant Staphylococcus aureus (MRSA). The chosen intravenous antibiotic targets the bacterial cell wall by binding to the D-alanyl-D-alanine terminus of cell wall precursor units. What is the most significant dose-limiting toxicity of this medication?

Explanation

The antibiotic described is Vancomycin, a glycopeptide that inhibits bacterial cell wall synthesis by binding to the D-ala-D-ala terminus of peptidoglycan precursors. The most significant and common dose-limiting toxicity of Vancomycin is nephrotoxicity, which requires trough level monitoring. While ototoxicity can occur, it is much rarer than renal impairment. Red Man Syndrome is a histamine-mediated infusion reaction, not a dose-limiting toxicity. Tendinopathy is associated with fluoroquinolones.

Question 34

Rank the following common orthopedic implant materials in order of decreasing Young's modulus (stiffness):

Explanation

Young's modulus is a fundamental biomechanical measure of the stiffness of a solid material. The correct order from stiffest (highest modulus) to most flexible (lowest modulus) is Alumina Ceramic (approx. 380 GPa) > Cobalt-Chrome alloys (approx. 210-240 GPa) > Stainless Steel (approx. 200 GPa) > Titanium alloys (approx. 100-110 GPa) > Cortical Bone (approx. 15-20 GPa) > PMMA bone cement (approx. 3 GPa) > Cancellous bone (approx. 0.1-2 GPa).

Question 35

The structural properties of a healing tendon or ligament are heavily dependent on the remodeling phase of tissue repair. During this phase, what is the primary cellular event responsible for increasing the tensile strength of the healing tissue?

Explanation

Tendon and ligament healing occurs in three overlapping phases: inflammation, proliferation, and remodeling. During the early proliferative phase, immature and mechanically weaker type III collagen is predominantly laid down. During the remodeling phase (which can last months to years), this type III collagen is gradually resorbed and replaced by stronger, more highly organized type I collagen. These type I collagen fibers cross-link and align parallel to the axis of mechanical stress, significantly increasing the ultimate tensile strength of the tissue.

Question 36

Romosozumab is a monoclonal antibody utilized in the treatment of severe osteoporosis to decrease the risk of fractures. What is its exact mechanism of action at the molecular level?

Explanation

Romosozumab is a monoclonal antibody that targets and inhibits sclerostin. Sclerostin is a glycoprotein secreted by osteocytes that normally downregulates bone formation by inhibiting the Wnt/beta-catenin signaling pathway in osteoblasts. By inhibiting sclerostin, romosozumab exerts a dual effect: it significantly promotes bone formation and also mildly decreases bone resorption, making it a highly effective anabolic agent for severe osteoporosis.

Question 37

Ligaments and tendons exhibit viscoelastic properties, making their mechanical behavior dependent on the rate of loading. Which of the following best describes the biomechanical phenomenon of 'creep'?

Explanation

Creep is a fundamental viscoelastic property defined as the progressive, time-dependent deformation of a material under a constant load (or stress). This is clinically relevant in casting and soft tissue stretching. 'Stress relaxation' is the decrease in stress over time when a material is held at a constant length or deformation. 'Hysteresis' is the loss of energy (usually as heat) between the loading and unloading curves.

Question 38

In the manufacturing of ultra-high-molecular-weight polyethylene (UHMWPE) for total joint arthroplasty, highly cross-linked polyethylene is typically subjected to a remelting process above its melting temperature (approximately 150°C). What is the primary purpose of this remelting step?

Explanation

Gamma irradiation is used to induce cross-linking in UHMWPE, which vastly improves its wear resistance. However, irradiation cleaves polymer chains and generates free radicals. If left untreated, these free radicals react with oxygen in vivo, leading to chain scission, oxidation, embrittlement, and catastrophic failure of the bearing. Thermal treatment (remelting or annealing) is performed to extinguish these free radicals. Remelting eliminates free radicals effectively but comes at the cost of a slight decrease in crystallinity and certain mechanical properties (like yield strength and fatigue resistance).

Question 39

Which of the following best describes the structural and biochemical characteristics of the superficial (tangential) zone of normal articular cartilage?

Explanation

The superficial (tangential) zone constitutes the top 10-20% of articular cartilage. It is characterized by the highest water content, the lowest concentration of proteoglycans, and the highest concentration of collagen (primarily Type II). The collagen fibers here are tightly packed and oriented parallel to the articular surface to resist shear and tensile stresses. The chondrocytes in this zone are flattened. The deep zone, in contrast, has the highest proteoglycan content, lowest water content, and collagen fibers oriented perpendicularly (radially) to resist compressive loads.

Question 40

Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant challenge in orthopedic implant-associated infections. Which genetic alteration is primarily responsible for the methicillin resistance in these organisms?

Explanation

Methicillin resistance in S. aureus is mediated by the staphylococcal cassette chromosome mec (SCCmec), which carries the mecA gene. The mecA gene encodes for an altered penicillin-binding protein, PBP2a, which has a remarkably low affinity for beta-lactam antibiotics. This allows the bacteria to continue cross-linking its peptidoglycan cell wall even in the presence of therapeutic levels of methicillin or other beta-lactams. VanA mediates vancomycin resistance, while rpoB mutations lead to rifampin resistance.

Question 41

A 65-year-old patient undergoes a revision total hip arthroplasty where a cobalt-chromium (CoCr) femoral head is placed onto a titanium alloy (Ti-6Al-4V) stem. Which of the following best describes the dominant type of corrosion that is uniquely facilitated by combining these two dissimilar metals?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in direct electrical contact within an electrolytic solution (such as body fluids). The less noble metal acts as the anode and preferentially corrodes. While CoCr and Titanium alloys are frequently paired in orthopedics due to their closely matched electrochemical potentials and passive oxide layers, mixing dissimilar metals inherently risks galvanic corrosion. Fretting and crevice corrosion also occur at modular junctions (like the trunnion) but are mechanically and geometrically driven phenomena that occur even in monobloc or mixed metal constructs.

Question 42

Bone morphogenetic proteins (BMPs) play a crucial role in fracture healing by inducing the differentiation of mesenchymal stem cells into osteoblasts. BMPs primarily exert their intracellular effects through which of the following signaling pathways?

Explanation

BMPs are members of the Transforming Growth Factor-beta (TGF-beta) superfamily. Upon binding to their specific serine/threonine kinase cell surface receptors, they trigger the phosphorylation of intracellular receptor-regulated Smad proteins (specifically Smad1, 5, and 8). These activated Smads form a complex with the common-mediator Smad4, which then translocates into the nucleus to regulate the transcription of osteogenic target genes, notably Runx2.

Question 43

A pediatric patient presents with unusually dense, fragile bones and is subsequently diagnosed with malignant infantile osteopetrosis. The pathogenesis of this disease is linked to the failure of osteoclasts to create an acidic environment in the sealing zone. Which enzyme is most commonly deficient or defective in this specific cellular process?

Explanation

Osteopetrosis is caused by defective osteoclastic bone resorption, resulting in excessively dense, brittle bones. To resorb bone, osteoclasts must acidify the sealed Howship's lacuna to dissolve the inorganic hydroxyapatite. This acidification relies heavily on Carbonic Anhydrase II (CA II), which catalyzes the conversion of water and carbon dioxide into carbonic acid, yielding the protons (H+) that are subsequently pumped into the lacuna by a V-ATPase pump (TCIRG1 mutation). Cathepsin K and TRAP are involved in degrading the organic matrix (collagen) after demineralization.

Question 44

Following a high-energy trauma, a patient exhibits a peripheral nerve injury characterized by loss of axonal continuity and endoneurial tube disruption, but with preservation of the perineurium and epineurium. According to the Sunderland classification, what grade is this nerve injury?

Explanation

According to the Sunderland nerve injury classification: First degree (Neuropraxia) involves local conduction block with intact axons. Second degree (Axonotmesis) involves axon disruption but an intact endoneurium. Third degree involves disruption of the axon and endoneurium, with an intact perineurium (intrafascicular scarring often blocks functional regeneration). Fourth degree involves disruption of the axon, endoneurium, and perineurium, leaving only the epineurium intact (neuroma-in-continuity). Fifth degree (Neurotmesis) is complete transection of the nerve.

Question 45

When applying a bridge plate to a comminuted diaphyseal fracture to promote secondary bone healing, increasing the 'working length' of the plate will have which of the following biomechanical effects?

Explanation

The 'working length' of a plate is defined as the distance between the two innermost screws closest to the fracture on either side. Increasing the working length decreases the stiffness of the construct in axial loading, bending, and torsion (decreased torsional rigidity). This calculated reduction in stiffness permits increased interfragmentary motion (micromotion), which is necessary to stimulate callus formation for secondary bone healing. It does not increase axial stiffness or rigid fixation.

Question 46

Which of the following enzymes is primarily responsible for the degradation of the organic bone matrix within the Howship's lacuna during osteoclastic bone resorption?

Explanation

Osteoclasts resorb bone by secreting hydrogen ions (via Carbonic anhydrase II and V-ATPase) to dissolve inorganic hydroxyapatite, and proteolytic enzymes like Cathepsin K to degrade the organic matrix (primarily Type I collagen). TRAP is a marker of osteoclasts and is involved in osteopontin dephosphorylation, but Cathepsin K is the primary protease responsible for collagen degradation at the ruffled border.

Question 47

The addition of Vitamin E (alpha-tocopherol) to highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) in total joint arthroplasty primarily serves to:

Explanation

Irradiation of UHMWPE creates cross-links that improve wear resistance but also generates free radicals that can lead to oxidative degradation and embrittlement over time. Vitamin E is added as an antioxidant to scavenge these free radicals, preventing oxidation without the need to melt or anneal the plastic, thereby preserving its mechanical properties and fatigue strength.

Question 48

In which zone of normal articular cartilage are the chondrocytes arranged in columns perpendicular to the joint surface, and what is the primary collagen type present?

Explanation

Articular cartilage is primarily composed of Type II collagen. In the deep zone, chondrocytes are arranged in vertical columns, and collagen fibers are oriented perpendicular to the joint surface to resist compressive forces. The superficial zone has collagen parallel to the surface to resist shear, while the calcified zone contains Type X collagen and hypertrophic chondrocytes.

Question 49

The pull-out strength of a cortical bone screw is most significantly increased by increasing which of the following design parameters?

Explanation

Screw pull-out strength is directly proportional to the outer (thread) diameter, the length of thread engagement in the bone, and the shear strength of the bone material, while being inversely proportional to the pitch. The outer diameter has the most mathematically significant effect on pull-out strength. The inner (core) diameter primarily determines the screw's bending strength, not pull-out strength.

Question 50

A 4-year-old boy presents with disproportionate short stature, rhizomelic shortening of the limbs, and frontal bossing. A mutation in the fibroblast growth factor receptor 3 (FGFR3) gene is identified. What is the molecular consequence of this mutation?

Explanation

Achondroplasia is caused by an activating (gain-of-function) mutation in the FGFR3 gene. Normally, FGFR3 negatively regulates bone growth by inhibiting chondrocyte proliferation and differentiation in the growth plate. The mutation causes constitutive activation of this receptor, leading to profound inhibition of endochondral ossification and resulting in disproportionate dwarfism.

Question 51

A randomized controlled trial compares a new NSAID to a placebo for postoperative pain after ACL reconstruction. The study fails to find a statistically significant difference between the two groups (p = 0.08), despite a true clinically significant difference actually existing in the general population. This scenario represents which of the following, and how could it have been prevented?

Explanation

A Type II error (false negative) occurs when a study fails to reject the null hypothesis when it is actually false (i.e., missing a true difference). This typically happens when a study is underpowered. Power is calculated as 1 - Beta (where Beta is the probability of a Type II error). Increasing the sample size is the standard way to increase statistical power and reduce the risk of a Type II error.

Question 52

Which of the following best describes the mechanism of action of teriparatide in the treatment of osteoporosis?

Explanation

Teriparatide is an anabolic agent and a recombinant human parathyroid hormone (PTH 1-34) analog. While continuous endogenous PTH elevation leads to bone resorption via osteoclast activation, intermittent (daily subcutaneous) administration of teriparatide preferentially stimulates osteoblastic bone formation, increasing bone mineral density and reducing fracture risk.

Question 53

Which of the following orthopedic biomaterials possesses a Young's modulus of elasticity most closely resembling that of human cortical bone?

Explanation

Young's modulus is a measure of material stiffness. Human cortical bone has a Young's modulus of approximately 15-20 GPa. Titanium alloy (approx. 100-110 GPa) is the closest metallic biomaterial to cortical bone compared to Stainless Steel (approx. 200 GPa) and Cobalt-Chromium (approx. 210-240 GPa). This closer modulus match helps reduce stress shielding around implants.

Question 54

During the process of secondary fracture healing, which type of collagen is predominantly synthesized during the soft callus phase, and which cells are primarily responsible for its production?

Explanation

Secondary fracture healing occurs via endochondral ossification. During the soft callus phase, the fracture gap is stabilized by cartilage formation in hypoxic conditions. Chondrocytes synthesize predominantly Type II collagen (along with aggrecan) to form this cartilaginous soft callus. Later, hypertrophic chondrocytes produce Type X collagen, the matrix mineralizes, and osteoblasts lay down Type I collagen to form the hard callus.

Question 55

A 65-year-old man develops a periprosthetic joint infection three months after a total knee arthroplasty. Staphylococcus epidermidis is isolated from the joint fluid. The pathogenesis of this infection involves the formation of a biofilm. Which of the following represents the critical first step in biofilm formation on the orthopedic implant?

Explanation

Biofilm formation begins with the immediate coating of the implant with host proteins (the conditioning film). Planktonic bacteria then undergo a phase of reversible attachment to this surface via van der Waals forces. This is followed by irreversible attachment (via adhesins like MSCRAMMs), proliferation, maturation via secretion of extracellular polymeric substance (EPS), quorum sensing, and eventual dispersion.

Question 56

An orthopaedic surgeon uses a stainless steel screw to secure a titanium plate during fracture fixation. Postoperatively, rapid degradation of the screw is observed at the interface with the plate. Which of the following best describes the primary mechanism of this implant degradation?

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in electrical contact within an electrolytic environment (such as bodily fluids). This establishes an electrochemical potential difference, causing the less noble (more anodic) metal to undergo accelerated corrosion. To prevent this, orthopaedic constructs should typically utilize components composed of the same metal or compatible alloys.

Question 57

Cortical bone exhibits viscoelastic properties, meaning its biomechanical behavior alters depending on the rate of loading. Which of the following statements best describes the response of cortical bone to a high strain rate (e.g., a high-velocity gunshot wound)?

Explanation

Bone is a viscoelastic and anisotropic material. At higher strain rates (rapid loading), bone becomes stiffer (has a higher modulus of elasticity) and its ultimate strength increases. Consequently, it can absorb more energy prior to failure compared to low loading rates. However, when the energy capacity is finally exceeded, the energy release often results in highly comminuted fractures with extensive soft tissue damage.

Question 58

Articular cartilage is structurally divided into four distinct zones. Which of the following best characterizes the composition and structure of the superficial (tangential) zone?

Explanation

The superficial (tangential) zone of articular cartilage makes up 10-20% of the cartilage thickness. It possesses the highest water content, the lowest concentration of proteoglycans, and a high concentration of collagen fibers (primarily Type II and IX) that are oriented parallel to the joint surface to maximally resist shear forces. The deep zone, in contrast, has the lowest water content, highest proteoglycan content, and vertically oriented collagen fibers to resist compressive loads.

Question 59

Monoclonal antibodies targeting sclerostin, such as romosozumab, are utilized in the management of osteoporosis. What is the primary molecular mechanism by which sclerostin normally influences bone remodeling?

Explanation

Sclerostin is a glycoprotein secreted primarily by mature osteocytes. It acts as a negative regulator of bone formation by binding to the LRP5/6 co-receptors on the surface of osteoblasts. This binding competitively blocks the Wnt/beta-catenin signaling pathway, leading to decreased osteoblastic activity and reduced bone formation. Inhibiting sclerostin results in a dual effect: increased bone formation and decreased bone resorption.

Question 60

Aseptic loosening secondary to periprosthetic osteolysis is a leading cause of total hip arthroplasty failure. This process is primarily mediated by a macrophage response to ultra-high-molecular-weight polyethylene (UHMWPE) wear debris. What is the optimal particle size range that most effectively stimulates macrophages to release pro-inflammatory cytokines?

Explanation

Macrophage-induced osteolysis is a particle-size-dependent phenomenon. The most biologically active UHMWPE wear particles are in the submicron range, specifically between 0.1 and 1.0 micrometers (often cited as 0.1-0.5 µm). These particles are easily phagocytosed by macrophages, which subsequently fail to digest them and instead release osteolytic cytokines like TNF-alpha, IL-1, and IL-6. Particles larger than 10 micrometers are typically engulfed by multinucleated giant cells but are less efficient at stimulating the massive cytokine cascade seen with submicron debris.

Question 61

Bone Morphogenetic Proteins (BMPs) belong to the TGF-beta superfamily and play critical roles in osteoinduction. Which of the following recombinant human BMPs is FDA-approved for use with an absorbable collagen sponge in the treatment of acute, open tibial shaft fractures?

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA-approved for application via an absorbable collagen sponge for acute, open tibial shaft fractures, as well as for anterior lumbar interbody fusions (ALIF) with certain interbody devices. rhBMP-7 (also known as Osteogenic Protein-1 or OP-1) previously had a Humanitarian Device Exemption for recalcitrant nonunions but is no longer commercially available. BMP-3 actually acts as an antagonist to bone formation.

Question 62

When evaluating prosthetic joint infections, bacteria within a mature biofilm exhibit significantly different survival characteristics compared to planktonic bacteria. Which of the following best describes the antimicrobial resistance profile of sessile bacteria within a biofilm?

Explanation

Bacteria residing in a biofilm (sessile state) are notoriously recalcitrant to antibiotic therapy and host immune clearance. They exist in a nutrient-depleted, stationary growth phase with low metabolic activity and are shielded by an extracellular polymeric substance (EPS) matrix. Because most antibiotics target actively dividing cells, sessile bacteria can survive antibiotic concentrations 100 to 1000 times higher than the Minimum Bactericidal Concentration (MBC) required to kill their planktonic (free-floating) counterparts.

Question 63

A patient sustains a closed fracture resulting in a peripheral nerve injury characterized by loss of axonal continuity and subsequent Wallerian degeneration. However, the endoneurium, perineurium, and epineurium remain completely intact. According to the Sunderland classification, what grade is this injury, and what is the expected clinical outcome?

Explanation

This injury describes a Sunderland Grade II nerve injury, which directly corresponds to Seddon's 'axonotmesis'. In a Grade II injury, the axon is disrupted leading to Wallerian degeneration distally, but the intact endoneurial tubes (along with the perineurium and epineurium) guide the regenerating axons. This architectural preservation leads to an excellent prognosis for complete spontaneous recovery, which typically proceeds at a rate of 1 mm per day (or 1 inch per month). Grade III involves disruption of the endoneurium, Grade IV involves disruption of the perineurium, and Grade V is a complete transection.

Question 64

Tranexamic acid (TXA) has become standard of care in numerous orthopaedic procedures for reducing perioperative blood loss and transfusion requirements. Which of the following accurately describes its primary pharmacological mechanism of action?

Explanation

Tranexamic acid (TXA) is a synthetic derivative of the amino acid lysine. It functions as an antifibrinolytic agent by reversibly binding to the lysine receptor sites on plasminogen. This competitive binding prevents plasminogen from interacting with fibrin, thereby inhibiting the activation of plasminogen into plasmin by tissue plasminogen activator (tPA). Consequently, fibrin degradation (fibrinolysis) is blocked, stabilizing the pre-existing clot.

Question 65

A 28-year-old male presents with a deep, slow-growing, painful soft tissue mass in the plantar aspect of his foot. Histopathological examination following core needle biopsy reveals a biphasic pattern consisting of both epithelial-like cells forming glandular structures and a spindle cell stromal component. Cytogenetic analysis is most likely to reveal which of the following specific chromosomal translocations?

Explanation

The clinical presentation (a deep, slow-growing soft tissue mass in a young adult's foot) and the classic biphasic histology (epithelial and spindle cells) are highly characteristic of synovial sarcoma. The pathognomonic molecular hallmark for synovial sarcoma is the t(X;18)(p11;q11) chromosomal translocation, which results in the SYT-SSX fusion gene. This translocation is found in >90% of cases. t(11;22) is associated with Ewing sarcoma; t(12;16) with myxoid liposarcoma; t(9;22) with extraskeletal myxoid chondrosarcoma; and t(2;13) with alveolar rhabdomyosarcoma.

Question 66

Ligaments and tendons exhibit time-dependent viscoelastic properties that are clinically relevant to orthopedics. Which of the following biomechanical phenomena describes the progressive deformation of a viscoelastic material when it is subjected to a constant load over a prolonged period?

Explanation

Creep is the time-dependent, progressive deformation of a viscoelastic material under a constant load (e.g., a serial cast applied to correct a clubfoot). Stress relaxation is the decrease in stress over time when a material is held at a constant length/strain. Hysteresis represents the energy lost (usually as heat) during the loading and unloading cycle of a viscoelastic material.

Question 67

A stress-strain curve representing a typical metallic orthopedic implant under tension is analyzed.

Which of the following accurately describes the 'yield point' on this curve?

Explanation

The yield point marks the transition from elastic deformation (where the material returns to its original shape when unloaded) to plastic deformation (where irreversible deformation occurs). The total area under the elastic curve is resilience. The maximum stress is the ultimate tensile strength. The linear slope represents Young's modulus of elasticity.

Question 68

A 68-year-old female with severe postmenopausal osteoporosis is started on teriparatide to reduce her fracture risk. Which of the following best describes the primary mechanism of action of this pharmacological agent?

Explanation

Teriparatide is a recombinant human parathyroid hormone (PTH 1-34) analog. When given via daily intermittent injections, it acts as an anabolic agent, preferentially stimulating osteoblast activity over osteoclast activity to build new bone. Denosumab binds RANKL. Bisphosphonates inhibit farnesyl pyrophosphate synthase and bind hydroxyapatite. Romosozumab inhibits sclerostin.

Question 69

Achondroplasia is the most common form of short-limbed dwarfism in humans. Which of the following best describes the underlying genetic mutation and its cellular consequence at the level of the physis?

Explanation

Achondroplasia is an autosomal dominant condition caused by an activating mutation (gain of function) in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. FGFR3 normally acts to inhibit endochondral ossification by suppressing chondrocyte proliferation in the proliferative zone of the physis. The activating mutation causes over-inhibition, leading to arrested long bone growth. Defective COMP is seen in pseudoachondroplasia and multiple epiphyseal dysplasia. CBFA1 mutations cause cleidocranial dysplasia.

Question 70

Orthopedic stainless steel implants (e.g., 316L) are composed primarily of iron, chromium, and nickel. What is the primary functional reason for adding molybdenum to this alloy?

Explanation

The 'L' in 316L stainless steel stands for low carbon, which prevents intergranular corrosion. The addition of molybdenum (usually 2-3%) specifically improves the alloy's resistance to pitting and crevice corrosion, which is critical in the chloride-rich physiological environment of the human body. Chromium provides a passivating oxide layer, and nickel stabilizes the austenitic face-centered cubic crystal structure.

Question 71

Articular cartilage is a highly specialized tissue structured in multiple histomorphologic zones.

Which of the following characteristics accurately describes the deep (basal) zone of healthy articular cartilage?

Explanation

The deep zone of articular cartilage is characterized by the highest concentration of proteoglycans and the lowest water content. The collagen fibers (mainly Type II) in this zone are large in diameter and oriented perpendicular to the joint surface to resist compressive loads. The superficial (tangential) zone has the highest water content, flattened chondrocytes, parallel collagen fibers, and provides resistance to shear forces.

Question 72

Highly cross-linked polyethylene (HXLPE) is widely used in total hip arthroplasty due to its superior wear characteristics. However, the process of cross-linking and subsequent remelting/annealing directly diminishes which of the following mechanical properties?

Explanation

While high-dose irradiation cross-linking of ultra-high-molecular-weight polyethylene (UHMWPE) dramatically reduces adhesive and abrasive wear, the cross-linking process and subsequent thermal treatments (remelting) significantly decrease mechanical properties such as fatigue strength, fracture toughness, and ultimate tensile strength. This makes HXLPE more vulnerable to rim cracking or failure under high-stress conditions or thin components.

Question 73

Tranexamic acid (TXA) is frequently administered intravenously and topically in major orthopedic surgeries to reduce perioperative blood loss. By which of the following mechanisms does TXA exert its therapeutic effect?

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It acts as an antifibrinolytic agent by reversibly and competitively binding to the lysine-binding sites on plasminogen. This prevents plasminogen from interacting with fibrin, thereby halting its activation into plasmin and preventing subsequent fibrin clot degradation.

Question 74

According to Perren’s strain theory of fracture healing, the type of tissue that forms in a fracture gap is determined by the local interfragmentary strain. Lamellar bone can only form when the interfragmentary strain is below which of the following thresholds?

Explanation

Perren's strain theory dictates that a specific tissue can only form and survive in a fracture gap if the interfragmentary strain is lower than the tissue's ultimate failure strain. Granulation tissue can tolerate up to 100% strain, fibrocartilage roughly 10-15%, and woven bone about 5%. Lamellar bone requires an extremely stable environment and can only form when the strain is less than 2%.

Question 75

In the pathogenesis of periprosthetic joint infection (PJI), Staphylococcus aureus utilizes the host implant surface to form a recalcitrant biofilm. Which stage of biofilm development is distinctly characterized by the production of a mature extracellular polymeric substance (EPS) matrix and the initiation of quorum sensing?

Explanation

Biofilm formation progresses through distinct phases: 1) Reversible attachment of planktonic bacteria. 2) Irreversible attachment mediated by adhesins. 3) Maturation, during which the bacteria multiply, form microcolonies, produce a dense extracellular polymeric substance (EPS) matrix, and engage in quorum sensing (chemical communication). 4) Dispersion (detachment), where bacteria escape to colonize new sites.

Question 76

A load-deformation curve of a bone specimen is shown in Figure 1.

Which of the following parameters represents the material's toughness?

Explanation

Toughness is defined as the amount of energy a material can absorb before it fractures, represented by the total area under the stress-strain curve. The slope of the linear elastic region represents the elastic modulus (stiffness). The peak load is the ultimate strength. The transition point from linear to non-linear is the yield point, marking the end of elastic deformation and the beginning of plastic deformation.

Question 77

Cross-linking of ultra-high-molecular-weight polyethylene (UHMWPE) components in total joint arthroplasty is primarily performed to improve which of the following properties?

Explanation

Highly cross-linked polyethylene (HXLPE) is irradiated to create free radicals that form cross-links, which significantly increases the material's abrasive wear resistance. However, this cross-linking process inherently decreases other mechanical properties, including tensile strength, fracture toughness, and fatigue resistance. Post-irradiation melting or annealing is done to extinguish remaining free radicals and prevent oxidation.

Question 78

In normal articular cartilage, the highest concentration of water and the orientation of collagen fibers parallel to the joint surface are found in which zone?

Explanation

The superficial (tangential) zone makes up 10-20% of articular cartilage thickness. It has the highest water content (approximately 80%), the lowest proteoglycan content, and features Type II collagen fibers arranged parallel to the joint surface to resist shear and tensile stresses. The deep zone has the lowest water content, highest proteoglycan content, and collagen fibers arranged perpendicular to the surface.

Question 79

A 4-year-old boy presents with short stature, frontal bossing, and rhizomelic shortening of the limbs. The genetic mutation responsible for this condition results in which of the following cellular level abnormalities?

Explanation

The clinical presentation is classic for achondroplasia, the most common form of short-limb dwarfism. It is caused by an autosomal dominant gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. FGFR3 normally acts as a negative regulator of bone growth. The mutation leads to constitutive activation of the receptor, which severely inhibits chondrocyte proliferation and hypertrophy in the proliferative zone of the physis.

Question 80

When managing a staphylococcal periprosthetic joint infection (PJI) with retention of the prosthesis, rifampin is frequently added to the antibiotic regimen due to its ability to penetrate biofilms. What is the mechanism of action of rifampin?

Explanation

Rifampin exerts its bactericidal effect by binding to the beta subunit of bacterial DNA-dependent RNA polymerase, thereby inhibiting RNA synthesis. It is uniquely effective against slow-growing or stationary-phase staphylococci embedded in biofilms. Option A describes beta-lactams. Option B describes aminoglycosides or tetracyclines. Option D describes daptomycin. Option E describes fluoroquinolones.

Question 81

Figure 2 illustrates a stage in the incorporation of a cortical bone allograft.

Which of the following is the defining characteristic of 'creeping substitution' during this process?

Explanation

Creeping substitution is the biological process by which a bone graft is incorporated by the host. It involves the simultaneous invasion of vascular granulation tissue, osteoclastic resorption of the dead graft trabeculae (via cutting cones in cortical bone), and osteoblastic deposition of new viable host bone onto those same dead trabeculae. It bypasses the cartilage phase seen in secondary bone healing (enchondral ossification).

Question 82

A patient sustains a closed midshaft humerus fracture and presents with a dense radial nerve palsy. Electromyography (EMG) performed at 4 weeks shows fibrillation potentials in the brachioradialis. High-resolution MRI reveals intact endoneurial and epineurial connective tissue architecture. According to the Seddon classification, this injury is best categorized as:

Explanation

Axonotmesis (Seddon classification) involves disruption of the axon and myelin sheath, leading to Wallerian degeneration distal to the injury (hence fibrillation potentials on EMG after 3-4 weeks). However, the supporting connective tissue structures (endoneurium, perineurium, epineurium) remain intact, allowing for potential spontaneous recovery as the axon regenerates along the intact tubes. Neuropraxia is a temporary conduction block without axonal disruption (no fibrillations). Neurotmesis is complete nerve transection.

Question 83

Tranexamic acid (TXA) is widely utilized in orthopedic surgery to reduce perioperative blood loss. Which of the following best describes its primary pharmacological mechanism?

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It acts as an antifibrinolytic agent by competitively and reversibly binding to the lysine-binding sites on plasminogen molecules. This prevents plasminogen from binding to fibrin, inhibiting its activation into plasmin and thereby preventing the degradation of fibrin clots (fibrinolysis).

Question 84

A 16-year-old boy presents with a painful, enlarging mass in the diaphysis of the femur. Biopsy reveals uniform, small, round blue cells. Cytogenetic analysis identifies a t(11;22)(q24;q12) chromosomal translocation. Which fusion gene is associated with this patient's diagnosis?

Explanation

The clinical presentation, histological appearance (small, round blue cells), and specific t(11;22)(q24;q12) translocation are diagnostic for Ewing sarcoma. This translocation results in the EWS-FLI1 fusion protein, an aberrant transcription factor that drives oncogenesis. SYT-SSX is associated with synovial sarcoma [t(X;18)]. TLS-CHOP (or FUS-DDIT3) is seen in myxoid liposarcoma [t(12;16)]. PAX3-FKHR is linked to alveolar rhabdomyosarcoma [t(2;13)].

Question 85

During the biomechanical testing of a native human tendon, the initial portion of the stress-strain curve exhibits a non-linear, concave upward slope prior to entering the linear elastic region. This specific portion of the curve, known as the 'toe region,' is primarily due to:

Explanation

The 'toe region' is the initial, non-linear segment of the stress-strain curve for ligaments and tendons where relatively little stress causes significant initial strain. Biologically, this corresponds to the 'un-crimping' or straightening of the naturally crimped (wavy) type I collagen fibers. Once the fibers are completely straight, the curve enters the linear elastic region. Microfailure occurs later, past the yield point in the plastic region.

Question 86

In a locking plate construct, decreasing the 'working length' (the distance between the two innermost screws on either side of the fracture) has which of the following biomechanical effects?

Explanation

The working length of a plate is defined as the distance between the two closest screws on opposite sides of the fracture. Decreasing the working length significantly increases the construct's overall stiffness (axial, bending, and torsional), thereby reducing interfragmentary strain. While highly stable, excessive stiffness may suppress callus formation and delay secondary bone healing in comminuted fractures.

Question 87

A patient presents with a painful, swollen thigh 5 years after an open reduction and internal fixation of a femur fracture. Radiographs show localized radiolucency around a stainless steel screw that was inadvertently placed through a titanium alloy plate. What is the primary mechanism of implant failure in this scenario?

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in direct physical contact within a conductive electrolytic fluid (such as body fluids). This creates an electrochemical potential difference, leading to an exchange of electrons and accelerated corrosion of the less noble metal (anode). This is why mixing dissimilar metals in a single construct is generally contraindicated.

Question 88

A pediatric patient with a diagnosis of achondroplasia presents for evaluation. At the cellular and molecular level within the physis, the gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene primarily leads to:

Explanation

Achondroplasia is an autosomal dominant disorder caused by a gain-of-function mutation in the FGFR3 gene. Normally, FGFR3 acts as a negative regulator of endochondral ossification. The mutation results in constitutive activation of the receptor, which severely inhibits chondrocyte proliferation and differentiation in the proliferative zone of the physis, resulting in short-limbed dwarfism.

Question 89

Which zone of normal articular cartilage is characterized by the highest water content, the lowest proteoglycan concentration, and collagen fibers oriented parallel to the joint surface?

Explanation

The superficial (tangential) zone of articular cartilage is the thinnest layer but is critical for resisting shear forces. It has the highest water content (approximately 80%), the lowest proteoglycan content, and densely packed type II collagen fibrils aligned parallel to the articular surface. Chondrocytes in this zone are flattened.

Question 90

Tranexamic acid (TXA) is widely utilized in orthopedic surgery to minimize perioperative blood loss. Which of the following best describes its mechanism of action?

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It exerts its antifibrinolytic effect by reversibly and competitively binding to the lysine receptor sites on plasminogen. This prevents plasminogen from binding to fibrin and being activated into plasmin, thereby inhibiting the degradation of fibrin clots (fibrinolysis).

Question 91

During tensile mechanical testing of a normal, healthy ligament, a characteristic stress-strain curve is generated. What structural phenomenon is responsible for the initial 'toe region' of this curve?

Explanation

The 'toe region' is the initial non-linear portion of the stress-strain curve for ligaments and tendons. It represents the elongation of the tissue with very little applied stress. This occurs due to the 'uncrimping' or straightening of the relaxed, naturally wavy collagen fibrils. Once the fibers are straight, the tissue enters the linear elastic region where stiffness increases.

Question 92

Osteoclasts resorb the inorganic hydroxyapatite matrix of bone by secreting hydrogen ions into the isolated microenvironment of the ruffled border. Which intracellular enzyme is primarily responsible for generating these hydrogen ions?

Explanation

Carbonic anhydrase II catalyzes the hydration of carbon dioxide to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate. The hydrogen ions are actively pumped across the ruffled border by vacuolar H+-ATPases to create an acidic environment (pH ~4.5) that dissolves bone mineral. Cathepsin K and MMPs are responsible for degrading the organic matrix (e.g., Type I collagen) after demineralization.

Question 93

In the pathogenesis of an orthopedic implant-associated infection, what is the critical first step in the development of a bacterial biofilm?

Explanation

Biofilm formation occurs in sequential stages. The initial step involves the reversible attachment of free-floating (planktonic) bacteria to the biomaterial surface via weak, non-specific forces (e.g., van der Waals forces, electrostatic interactions). This is followed by irreversible attachment mediated by specific adhesins (like MSCRAMMs), proliferation, EPS matrix production, maturation (involving quorum sensing), and finally dispersion.

Question 94

A 68-year-old patient is prescribed a postoperative oral anticoagulant following total hip arthroplasty. If the chosen medication acts as a direct, highly selective, reversible inhibitor of Factor Xa, which drug was prescribed?

Explanation

Rivaroxaban (and apixaban) are direct, selective, and reversible inhibitors of Factor Xa. Dabigatran is a direct thrombin (Factor IIa) inhibitor. Warfarin is a vitamin K antagonist affecting factors II, VII, IX, and X. Enoxaparin is a low-molecular-weight heparin that binds antithrombin III to inhibit Factor Xa indirectly. Fondaparinux is a synthetic pentasaccharide that also indirectly inhibits Factor Xa via antithrombin III.

Question 95

When comparing different skeletal muscle fiber types, fast-twitch glycolytic fibers (Type IIb) possess which of the following physiological and structural characteristics relative to slow-twitch oxidative fibers (Type I)?

Explanation

Type IIb fibers (fast-twitch glycolytic) are specialized for rapid, high-force contractions but fatigue quickly. They have low myoglobin content (appearing 'white'), a low capillary density, and fewer mitochondria. They rely heavily on anaerobic glycolysis for ATP production, hence their high glycolytic enzyme capacity. Type I fibers are 'red', fatigue-resistant, mitochondria-rich, and rely on oxidative phosphorylation.

Question 96

Sclerostin is an important regulator of bone metabolism and the target of the osteoporosis medication romosozumab. Which of the following best describes the primary mechanism of action of sclerostin?

Explanation

Sclerostin is a glycoprotein secreted primarily by mature osteocytes that acts as a potent negative regulator of bone formation. It functions by binding to the LRP5/6 co-receptors on the surface of osteoblasts, which competitively inhibits the canonical Wnt/β-catenin signaling pathway. This pathway is essential for osteoblast differentiation, proliferation, and survival. Inhibition of sclerostin (e.g., via the monoclonal antibody romosozumab) allows Wnt signaling to proceed, leading to increased bone formation.

Question 97

Which of the following modifications to ultra-high molecular weight polyethylene (UHMWPE) used in total joint arthroplasty most effectively reduces adhesive and abrasive wear while preserving oxidation resistance?

Explanation

Highly cross-linked polyethylene (HXLPE) is manufactured using gamma or electron beam irradiation, which breaks polymer chains and facilitates cross-linking. This significantly improves wear resistance, reducing adhesive and abrasive wear. However, irradiation creates free radicals that can react with oxygen over time, leading to oxidative degradation and embrittlement. To combat this, the polyethylene is often remelted (heated above its melting point) to extinguish these free radicals, thereby preserving oxidation resistance, although remelting can cause a slight decrease in fatigue strength compared to annealing (heating below the melting point).

Question 98

A 65-year-old patient is scheduled for a total knee arthroplasty and will be prescribed rivaroxaban for postoperative venous thromboembolism (VTE) prophylaxis. At which specific point in the coagulation cascade does this medication primarily exert its effect?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants (DOACs) that function by directly and competitively inhibiting Factor Xa, interrupting both the intrinsic and extrinsic pathways of the blood coagulation cascade and preventing the conversion of prothrombin to thrombin. Dabigatran (Option A) is a direct thrombin inhibitor. Low molecular weight heparins like enoxaparin (Option B) act indirectly via antithrombin III. Warfarin (Option D) inhibits vitamin K epoxide reductase, and aspirin (Option E) irreversibly inhibits COX-1.

Question 99

During internal fixation of a diaphyseal fracture, a surgeon aims to maximize the pull-out strength of a cortical screw. Which of the following alterations in screw design or insertion technique most significantly increases the screw's pull-out strength?

Explanation

The pull-out strength of a screw is a measure of its resistance to axial pull-out forces. It is directly proportional to the outer (thread) diameter, the length of engagement in the bone, and the volume of bone caught between the threads. Decreasing the thread pitch (the distance between adjacent threads) increases the number of threads engaged per unit of length, which subsequently increases the pull-out strength. Increasing the inner (core) diameter while maintaining the outer diameter decreases the thread depth, which would reduce pull-out strength (though it increases the screw's torsional strength).

Question 100

During the pathogenesis of a periprosthetic joint infection (PJI), Staphylococcus epidermidis transitions from a planktonic state to form a robust mature biofilm on the implant surface. Which of the following components is the primary structural constituent of the extracellular polymeric substance (EPS) matrix synthesized by these bacteria during the accumulation phase?

Explanation

Biofilm formation in Staphylococcus epidermidis (and many S. aureus strains) heavily relies on the production of Polysaccharide Intercellular Adhesin (PIA), which is synthesized by the gene products of the icaADBC operon. PIA forms the main bulk of the extracellular polymeric substance (EPS) matrix during the accumulation phase, encasing the bacteria and protecting them from host immune responses and antimicrobial agents. Fibronectin-binding proteins (Option D) are critical for the initial adhesion phase to host proteins coating the implant, rather than forming the bulk of the mature matrix.

AAOS & ABOS Basic Science MCQs (Set 3): Bone Biology, Biomechanics & Anatomy Review

Orthopedic Basic Science 2026 MCQs: Board Review Questions & Answers (Part 4)

Key Takeaway

Orthopedic Basic Science 2026 MCQs: Board Review Questions & Answers (Part 4)

Chapter Index